1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus having such a sheet feeding apparatus, and more particularly, it relates to a sheet feeding apparatus in which a sheet is fed out by sheet feeding means provided above the sheet for lifting and lowering movements.
2. Related Background Art
In some of conventional image forming apparatuses such as printers, copying machines and the like, there is provided a sheet feeding apparatus in which a sheet is fed to an image forming portion by sheet feeding means such as a pick-up roller. Among such sheet feeding apparatuses, there is a sheet feeding apparatus in which the sheet feeding means is provided above the sheet for lifting and lowering movements in such a manner that, when the sheet is fed, the sheet feeding means is lowered to be urged against the sheet and, in this condition, the sheet is fed out by rotating the sheet feeding means, and, thereafter, the sheet feeding means is lifted to be separated from the sheet.
By the way, in such a sheet feeding apparatus, although means for lifting and lowering the sheet feeding means was generally comprised of a solenoid or a cam one revolution of which is controlled, recently, as the speed of the printer has been increased, it is required that the sheet feeding means be urged against the sheet at higher speed and also be separated from the sheet at higher speed after the feeding of the sheet. Further, as noise of recent printers has been reduced, it is required for avoiding usage of an actuator such as a solenoid generating great noise.
To this end, there has been proposed a sheet feeding apparatus in which, as the means for lifting and lowering the sheet feeding means, a lifting and lowering mechanism for directly lifting and lowering the sheet feeding means by using a pulse motor, for example, is provided.
FIG. 8 shows a construction of such a conventional sheet feeding apparatus using a pulse motor. In FIG. 8, the sheet feeding apparatus comprises pick-up rollers 1a, 1b as sheet feeding means provided for lifting and lowering movements, and rotations of the pick-up rollers around shafts 2a, 2b are controlled by drive sources (not shown). Further, the pick-up rollers 1a, 1b are rotatably held on ends of roller holders 3a, 3b which are held by an image forming apparatus (not shown) for rotations around shafts 4a, 4b. 
Incidentally, there are provided roller springs 5a, 5b as urging means for biasing the pick-up rollers 1a, 1b in anti-clockwise directions (along which the rollers are urged against an upper surface of a stached sheet P) via the roller holders 3a, 3b so that the pick-up rollers 1a, 1b are urged against the sheet (not shown) with predetermined pressure by biasing the pickup rollers 1a, 1b in the anti-clockwise directions by means of the roller springs 5a, 5b. 
Further, in FIG. 8, the reference numeral 6 denotes a rod as a holding member capable of moving in an up-and-down direction and adapted to hold the pick-up rollers 1a, 1b for lifting and lowering movements, and xe2x80x9cMxe2x80x9d denotes a pulse motor as a reversible motor. Rotation of the pulse motor M is transmitted to the rod 6 via a motor gear 12, a first drive transmitting gear 11 as a first transmitting gear meshed with the motor gear 12, a second drive transmitting gear 13 as a second transmitting gear integrally formed with the first drive transmitting gear 11, and a rack gear 10 meshed with the second drive transmitting gear 13 and provided on one side of the rod 6. Incidentally, the first and second drive transmitting gears 11, 13 are rotatably held on a rotary shaft 17.
On the other hand, guide holes 6c extending in an up-and-down direction are formed in upper and lower portions of the rod 6, so that, when the rotation of the pulse motor M is transmitted via the gear train comprised of the motor gear 12, first and second drive transmitting gears 11, 13 and rack gear 10, the rod 6 is shifted in the up-and-down direction while being guided by pins 7, 8 inserted in the guide holes 6c. 
Incidentally, support portions 6a, 6b for supporting the shafts 2a, 2b of the pick-up rollers 1a, 1b from below are protruded horizontally from lower and upper ends of the rod 6. With this arrangement, when the rod 6 is shifted in the up-and-down direction, the pick-up rollers 1a, 1b are shifted in the up-and-down direction by the aid of the spring forces of the roller springs 5a, 5b or in opposition to the spring forces as the rod 6 is shifted.
Next, a sheet feeding operation of the sheet feeding apparatus having the above-mentioned construction will be explained.
In a waiting condition of the sheet feeding apparatus before it starts the sheet feeding operation, the rod 6 is held at a highest position or home position, so that the pick-up rollers 1a, 1b are positioned above the sheet. In this case, although the pulse motor M is not rotated, it is maintained in an exciting condition so that the rod 6 is held in the home position in opposition to the biasing forces of the roller springs 5a, 5b. 
On the other hand, when sheet feeding command is emitted from a controlling device (not shown) provided in the image forming apparatus, first of all, the pulse motor M is rotated in the anti-clockwise direction, and the anti-clockwise rotation is transmitted to the rod 6 via the motor gear 12, first and second drive transmitting gears 11, 13 and rack gear 10, with the result that the rod 6 is lowered. When the rod 6 is lowered in this way, the pick-up rollers 1a, 1b are also lowered together with the rod 6 by the biasing forces of the roller springs 5a, 5b, thereby urging the pick-up rollers against the sheet P.
Incidentally, even after the pick-up rollers 1a, 1b are urged against the sheet in this way, the rod 6 is further lowered by a predetermined distance. Here, when the rod 6 is lowered in this way, since the pick-up rollers 1a, 1b abut against the sheet, the support portions 6a, 6b of the rod 6 are separated from the shafts 2a, 2b of the pick-up rollers 1a, 1b. As a result, the pick-up rollers 1a, 1b are urged against the sheet P with predetermined abut pressure by the biasing forces of the roller springs 5a, 5b. 
After the pick-up rollers 1a, 1b are urged against the sheet P in this way, by rotating the pick-up rollers 1a, 1b, the sheet P can be fed to the image forming portion (not shown).
On the other hand, when the sheet feeding operation is finished, the pulse motor M is rotated in a clockwise direction, with the result that the rod 6 is lifted and the support portions 6a, 6b abut against the shafts 2a, 2b of the pick-up rollers 1a, 1b from below. Further, thereafter, when the rod 6 is lifted, the pick-up rollers 1a, 1b are lifted together with the rod 6 in opposition to the biasing forces of the roller springs 5a, 5b, thereby separating the pick-up rollers from the sheet P. Thereafter, the rod 6 is returned to the home position. In this case, by bringing the pulse motor M to the exciting condition, the rod 6 is held at the home position in opposition to the biasing forces of the roller springs 5a, 5b. 
By the way, the during such a sheet feeding operation, since the pick-up rollers 1a, 1b are biased so that they generally abut against the sheet P with load in the order of about 0.5 N to about 3 N, when the above-mentioned sheet feeding operation is effected, torque acting on the pulse motor M is changed as shown in FIG. 9. Incidentally, in FIG. 9, the ordinate indicates the position of the rod 6 and the abscissa indicates torque acting on the motor shaft of the pulse motor M.
As apparent from FIG. 9, when the rod 6 is in the home position, maximum torque Tmax acts on the pulse motor M to hold the rod 6 in the home position. Further, at the time when the pick-up rollers 1a, 1b are contacted with the sheet, although the torque is instantaneously decreased by an amount corresponding to the roller pressure, even in a condition HL that the rod 6 is lowered at the maximum extent, torque Tmin corresponding to the weight of the rod 6 itself acts on the pulse motor M.
Accordingly, in the conventional sheet feeding apparatus using such a pulse motor M, the electrical power is required even in the waiting condition, and, since the maximum torque is great, a large torque motor is required, which is very disadvantageous in consideration of the power consumption.
On the other hand, proper backlash is provided between the first drive transmitting gear 11 and the motor gear 12. Further, as shown in FIG. 10, proper backlash B is also provided between the second drive transmitting gear 13 and the rack gear 10 of the rod 6.
However, in the conventional sheet feeding apparatus having the lifting and lowering means including such a pulse motor M and the gear train for transmitting the rotation of the pulse motor M to the rod 6, since the pulse motor M must quickly be started and stopped reversibly and slow-up and slow-down control is effected on demand, due to the backlash B in the gears 10 to 13, great discordant slapping noise is generated in meshed portions in the gear train, which makes reduction of noise difficult or impossible.
The present invention is made in consideration of the above-mentioned circumstances, and an object of the present invention is to provide a sheet feeding apparatus which can reduce power consumption and noise, and an image forming apparatus having such a sheet feeding apparatus.
To achieve the above object, the present invention provides a sheet feeding apparatus comprising sheet feeding means for abutting against an upper surface of a sheet stacked and feeding out the sheet, biasing means for biasing the sheet feeding means toward the upper surface of the sheet stack, and lifting and lowering means for lifting and lowering the sheet feeding means, wherein the lifting and lowering means comprises a holding member for engaging with the sheet feeding means and shifting the sheet feeding means in an up-and-down direction by a motor, and maintaining means for regulating the holding member so as to maintain the sheet feeding means at a position where the sheet feeding means is spaced apart from the upper surface of the sheet, in opposition to a biasing force of the biasing means.